Linearization of inherently non-linear devices is necessary if distortion of signals through the device is to be minimized. Specifically when it concerns amplifiers in wireless communications it is in most cases necessary to limit spurious signal generation at out-of-band frequencies and also in some cases at in-band frequencies. One of the most frequently used techniques involves subtracting the unwanted signal portion from the output of the device by a feed-forward technique. The non-distorted input signal is then subtracted from an attenuated copy of the distorted output signal to give an estimate of the added spurious signal components, which may be again subtracted from the output signal by proper amplification and phase alignment. This technique is referred to as “Feed Forward” techniques, FF.
Another method to compensate for unwanted signal components is to find a mathematical operator which, if applied to the input signal, gives enough anti-distortion to compensate for the non-linearities of the device itself. This operator will if placed in a cascade configuration cancel out the unwanted signal components at the non-linear device output. This technique is referred to as “Pre-Distortion” techniques, PD.
A specific application of non-linear pre-distortion is to combat non-linear effects in power amplifiers for cellular communications. As capacity will drive the evolution of existing and forthcoming standards to deliver a higher number of user channels per bandwidth, amplifiers will have to be used in a broadband fashion. That is, if fed through a non-linear amplifier, these channels or frequencies would interfere with each other and also produce unwanted spurious signals in other frequency bands which are used by other network providers or mobile users.
A realization that uses the previously mentioned FF technique is the Multi Carrier Power Amplifier (MCPA). This device incorporates delicate tuning circuits for amplitude and phase matching so that only the distortion is subtracted from the output signal. A way to strengthen the linearization would be to cascade a pre-distorter or alternatively linearize the internal amplifier itself. In that process, it is expected that the overall efficiency will be increased. Another way is to take away the FF circuitry and solely rely on a good pre-distorter and good internal amplifier. It is believed that this would decrease manufacturing cost and also provide a simpler overall design.
State-of-the-art of pre-distortion compares samples of the signals measured at the input and output ports of a non-linear device, and then compensates by amplifying or attenuating the specific amplitude level. In this way, a non-linear amplitude characteristic function is obtained by which non-linear effects may be compensated for. Phase adjustment can be made in a similar way.
Another method may incorporate a more detailed algorithm whereby the characteristics of the non-linear device are tracked by time-dynamic methods such as expansion of the Volterra series, in order to find a closed expression for the device. The next step would then be to find an inverse to this function by the same expansion and apply it as a pre-distorter.